While muscle pull on flexor or extensor provides a turning moment to facilitate joint motion, the stability of a joint in either a still position or motion depends on both the constraints of the natural bicondylar joint structure and the integrity of the soft tissue mechanisms. Typical joint motions include restricted flexion and rotational and lateral deviation by constraints from the bicondylar joint configuration and ligamentous support. For example, the normal motion range of a proximal interphalangeal (PIP) joint is around 0° in extension to 100° in full flexion with restricted rotational and lateral deviation. The motion of a PIP joint is similar to that of a single axis hinge joint, in which the rotation axis is parallel to an articulating surface allowing the joint to flex in a uniplanar motion. Mobility of finger joints especially in metacarpophalangeal (MCP) and PIP joints is critical for providing normal grasp and pinch for various hand functions in daily activities.
Injuries such as bone fracture or joint damage can lead to loss of joint function, pathological disturbance by ostcoarthritis, posttraumatic arthritis and rheumatoid arthritis. Affected joints usually end up with erosions in the articular cartilage and bone resulting in unstable bony support, synovitis and inflammatory responses, which can destroy surrounding soft tissue. Consequences from progressive joint destruction are pain, joint instability, loss of mobility, excessive deformity and unrecoverable disability of hand function from soft tissue rupture.
Surgical intervention by implanting arthroplasty has been implemented to restore destroyed or deformed articulations to thus relieve pain, restore joint motion, correct deformity, and improve motion function prior to any salvage procedure of arthrodesis or removal of the joint. Artificial joint prostheses have been used in large joint replacement. However, small joint arthroplasty in particular finger joint replacement has not achieved the same results. The inability to reproduce the complex anatomical structure of the joint and the small size of joint components presents long-term clinical problems and renders surgical procedures difficult.
U.S. Pat. No. 3,466,669 discloses a fully constrained hinge type prosthesis, which can restore some joint function, in particular relieve pain. However, such fully constrained hinge type prosthesis cannot restore the normal arc of rotation. Consequently, excessive stress can exert on the bone implant interface resulting in progressive bone resorption with consequent prosthetic loosening and bone penetration.
U.S. Pat. Nos. 3,462,765 and 5,824,095 disclose one piece hinge type designs, using flexible silicon material. But there are concerns in the long-term performance of these implants since they can restore only a limited range of motion of around 30–40 degrees in flexion. In addition, both silicon breakages and silicon-induced synovitis from silicon wear particles are frequently reported.
The problems arising from constrained implants were dealt with by finger joint prostheses with semi-constrained articulation, such as designs from RMS and DJOA, increase the post-operative range of motion. In such designs, the articulating surfaces are modeled to closely resemble the anatomical shape of a real joint with no linking inbetween. The joint stability thus relies mostly on the integrity of the surrounding soft tissue, such as the collateral ligament and the volar plate. However, due to muscular imbalance, a potential recurrence of joint deformity still exists. Moreover, aseptic loosening stimulated by induced particles originated from polyethylene components is still unsolved in joint arthroplasty.
The present invention provides a prosthetic device that is capable of reducing complications and deficiencies arising from previous finger joint prostheses. For example, the prosthetic device of the invention is capable of providing efficient pain relief, increased movement range, enhanced fixation, intrinsic joint stability, and extended durability. The invention also provides an improved surgical technique by using a surgically implantable device which substantially resembles the original anatomical structure of a finger joint so that other surgical reconstructions to the adjacent soft tissue structures can be carried out at the same time.